Compared to conventional magnetic recording, optical recording, which is capable of noncontact recording and reproducing, is characterized by being less influenced by marring or fouling and is contributing greatly to an increase in storage capacity.
Recording media for use in this technique are constituted by forming an information recording layer on a transparent substrate made of, e.g., a polycarbonate resin. Polycarbonate resins are suitable for use as the material of substrates for the information recording media because they have satisfactory heat resistance in melt molding, reduced dimensional changes after molding, and excellent mechanical properties. In recent years, with the increasing storage capacity in this field where polycarbonate resins are used, the distance between pit tracks transferred to a transparent substrate is becoming shorter and the depth of the pits are becoming larger. As a result, it has become usual to mold polycarbonate resins at higher molding temperatures and higher mold temperatures. However, because of the insufficient heat resistance of the resins, molding at higher temperatures tends to yield low-molecular volatiles, which deposit on the stamper and replicas to cause bit errors, leading to serious problems. The higher-temperature molding further causes scorching due to resin deterioration, resulting in an increased error frequency in the final products. Because of such various problems, there has been a desire for a material which can be molded at a lower temperature. On the other hand, the mold temperature also is becoming higher so as to cope with the shorter distance between bit tracks and the larger depth of pits. However, as the mold temperature increases, the problem of warpage of molded substrates becomes severer. There has hence been a desire for a lower mold temperature as in the case of molding temperature.
Under these circumstances, polycarbonate resins have been strongly required to be a material which can be molded at a lower molding temperature and a lower mold temperature without impairing important properties possessed by disk substrates (birefringence, suitability for transfer, and warpage).
It has been well known in this respect that a polycarbonate resin which itself has improved flowability is obtained by using an alkylphenol in which the alkyl chain is longer than in the alkylphenols conventionally used as chain terminators for polycarbonate resins. For example, British Patent 965,457 discloses a technique of lowering melt viscosity by using a long-chain monohydric alcohol as a chain terminator, while U.S. Pat. No. 3,240,756 discloses the use of an alkylphenol as a terminator. In Unexamined Published Japanese Patent Application No. 51-34992 are given examples in which a phenol, acid chloride, acid, or alcohol having an alkyl group having 8 to 20 carbon atoms is used as a chain terminator to produce a polycarbonate having improved flowability. Recently, examples in which a polycarbonate resin having a long-chain alkyl group at each terminal is used as an optical molded article are given in Unexamined Published Japanese Patent Application No. 60-203632. The above references each states that an improvement in flowability is possible due to the long-chain alkyl groups present at molecular ends of the polycarbonate.
However, it has been found that such a polycarbonate obtained with a long-chain alkylphenol gives products having a far more yellowish hue as compared with those obtained from the polycarbonates produced with t-butylphenol, which has been conventionally employed, and that the polycarbonate is colored to such a degree that the molded products are unsuitable for use in optical recording media.